For example, JPH07-270711A teaches a head-up display apparatus, in which a bundle of rays of light is diffused by a screen member, and the bundle of rays of light, which is diffused by the screen member is projected as a display image onto a display member to enable a viewer to see a virtual image of the display image.
The screen member of the head-up display apparatus of JPH07-270711A is an optical member, in which a plurality of micro-optical elements is arranged in a grid pattern to transmit or reflect a bundle of rays of light that is incident on the respective micro-optical elements. The micro-optical elements are generally referred to as micro-lenses or micro-mirrors, and each of the micro-lenses or micro-mirrors is configured into a rectangular form in a view taken in a direction perpendicular to a plane of the screen member. These micro-lenses or micro-mirrors are arranged in the grid pattern to form a micro-lens array or a micro-mirror array. A surface of each of the micro-lenses or micro mirrors is formed as a curved surface having a predetermined radius of curvature to diffuse the bundle of rays of light, which is incident onto the curved surface, at a predetermined angle of diffusion. Therefore, the bundle of rays of light, which is outputted from the screen member at the predetermined angle of diffusion and is projected onto the display member, is guided to a predetermined range that is set around eyes of the viewer. Thereby, even when the eyes of the viewer are moved in the predetermine range in the vertical direction and the horizontal direction of the vehicle, the viewer can visually recognize the virtual image of the display image. This predetermined range is generally referred to as an eye box,
JPH07-270711A further teaches adjustment of a width and a height of each of the micro-lenses or micro-mirrors. Specifically, JPH07-270711A teaches adjustment of a size of the eye box by adjusting pitches of the micro-lenses or micro-mirrors in the direction of the width and the direction of the height of each of the micro-lenses or micro-mirrors.
In the head-up display apparatus, in order to enable the viewer to visually recognize the virtual image of the display image even in a case where the viewer largely moves his/her head to check, for instance, his/her surrounding at the time driving the vehicle, it may be desirable to form sub-eye boxes next to the eye box, in which the virtual image can be visually recognized by the viewer. The sub-eye box is a range, in which the virtual image is visually recognizable with the minimum brightness. In the following discussion, the eye box, which is formed next to the sub-eye box, is referred to as a main eye box.
However, when pitches of the micro-lenses or micro-mirrors in the direction of the width and the direction of the height are adjusted to enable the visual recognition of the virtual image in the sub-eye box, the bundle of rays of light, which impinges on the screen member, is uniformly guided to both of the main eye box and the sub-eye boxes. This is considered to be a result of a uniform setting of a radius of curvature of the curved surface of the respective micro-lenses or micro-mirrors.
When the viewing range is formed in the above-described manner, the energy of the bundle of rays of light, which is incident on the screen member, is also supplied to the sub-eye boxes. Therefore, the quantity of light in the main eye box is reduced in comparison to the case where only the main eye box is set as the viewing range, and thereby the brightness of the virtual image, which is viewed in the main eye box, is reduced in comparison to the case where only the main eye box is set as the viewing range. Furthermore, the quantity of light in the sub-eye box becomes substantially equal to the quantity of light in the main eye box. Therefore, the quantity of light in the sub-eye box is excessively increased, and thereby the brightness of the virtual image in the sub-eye box is excessively increased. As a result, the use efficiency of the bundle of rays of light is disadvantageously deteriorated.